1. Field of the Invention
The invention generally relates to integration of a switching function of a transmit module on bulk CMOS, and more particularly to such switching function while limiting the swing seen on the MOS transistors used by the switching function.
2. Prior Art
Transmit modules (TXMs) are increasingly the dominant front-end for GSM and EDGE phones. An exemplary TXM 100 is shown in FIG. 1. These TXMs contain power amplifiers (PAs), for example PAs 110 and 120, some (passive) matching circuitry coupled to the PAs, for example matching circuitries 130 and 140 respectively, filtering circuitry coupled to the matching circuitry, for example filtering circuitry 150 and 160 respectively, and a switching function coupled to the filtering circuitry, for example switching function 170. Typically there are two switching functions, one that switches between the transmit function and receive function (T/R switch) and another one that switches the band in multi-band phones (band switching). In most TXMs, these functions are combined in a single switch. For a quad band GSM TXM, the front-end switch 170 requires six poles. Two of these poles are used to switch the low band (LB)PA and high band (HB) PA in and out, i.e., the switch being open unless the respective PA is being used. Four switches are used to switch between the four receive (Rx) bands, i.e., open unless the particular band is selected. This puts a switch in the transmit path, this switch having to be able to handle a large voltage swing and high current. The switch will also contribute at least some power loss to the TXM 100.
In gallium arsenide (GaAs) based PAs, a separate switch is usually needed as the GaAs process used for the PA is a heterojunction bipolar transistor (HBT) process that is not suitable for making switches. In bulk complimentary metal-oxide semiconductor (CMOS), the transistors make good switches at low voltages, but at high voltages the transistors suffer from distortion and breakdown issues. Switches in TXMs are therefore usually made using GaAs pseudomorphic high electron mobility transistors (pHEMTs) or from semiconductor on insulator (SOI) CMOS which is known to be different from bulk CMOS.
Notably, the PA function has been moving away from older GaAs-based implementations, with one or two GaAs dies and a CMOS controller chip, to a single CMOS die to cover power output and control functionality. However, the switch is difficult to implement in bulk CMOS because of the large voltage swings (33 dBm into 50Ω which is 10V RMS swing) at the antenna port. The problems with the large voltage swing for a bulk CMOS process include the possibility of breakdown of the MOS oxide or drain and source diodes, as well as the reverse biased diode to substrate which can be forward biased and has a non-linear capacitor associated with it that generates unwanted harmonic interference.
In view of the deficiencies of the prior art, it would be advantageous to provide a solution that enables avoiding the need for a switch in the transmit path.